1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor which is used in, for example, an electrophotographic image forming apparatus of a copying machine or the like.
2. Description of the Related Art
An electrophotographic image forming apparatus has found wide acceptance in not only a copying machine but also a printer, an output device of a computer which has been increasingly demanded in recent years. In the electrophotographic image forming apparatus, a photoreceptive layer of an electrophotographic photoreceptor installed in the apparatus is uniformly charged with a charging unit, exposed to, for example, a laser beam corresponding to an image information, and a fine-grain developer called a toner is supplied to an electrostatic latent image formed by the exposure from a developing unit to form a toner image.
The toner image formed by adhering the toner as a component of the developer to the surface of the electrophotographic photoreceptor is transferred onto a transfer member such as a recording paper with a transfer unit. However, the toner on the surface of the electrophotographic photoreceptor is not completely transferred onto a recording paper, but a part thereof remains on the surface of the electrophotographic photoreceptor. Further, a paper powder of the recording paper contacted with the electrophotographic photoreceptor in the development might remain on the electrophotographic photoreceptor while being adhered thereto.
Since the remaining toner and the adhered paper powder on the surface of the photographic photoreceptor have an adverse effect on a quality of an image formed, they are removed with a cleaner. Moreover, in recent years, a cleanerless technique has been advanced, and the remaining toner is removed by such a development/cleaning system that it is recovered by a cleaning function imparted to a developing unit without providing an independent cleaning unit. Since charging, exposure, development, transfer, cleaning and electricity removal are repeatedly performed in the electrophotographic photoreceptor, durability to electrical and mechanical outer forces is required. Specifically, the electrophotographic photoreceptor requires durability to abrasion or damage caused by rubbing the surface of the electrophotographic photoreceptor, deterioration of a surface layer by adhesion of active substances such as ozone and Nox generated in charging with a charging unit, and the like.
In order to realize cost reduction and maintenance-free condition of the electrophotographic image forming apparatus, it is important that the electrophotographic photoreceptor has satisfactory durability and can be operated stably for a long period of time. One of factors that influence the durability and the long-term stability of the operation is surface cleanability, namely, ease of surface cleaning which is related with the surface condition of the electrophotographic photoreceptor.
The cleaning of the electrophotographic photoreceptor means that a force exceeding adhesion between the surface of the electrophotographic photoreceptor and the remaining toner or paper powder adhered is exerted on the remaining toner or paper powder to remove the adherent matter from the surface of the electrophotographic photoreceptor. Accordingly, the lower the wettability of the surface of the electrophotographic photoreceptor, the easier the cleaning. The wettability, namely, the adhesion of the surface of the electrophotographic photoreceptor can be expressed using a surface free energy (which has the same meaning as a surface tension) as an index.
The surface free energy (γ) is a phenomenon which an intermolecular force, a force acting between molecules constituting a substance, causes on the outermost surface.
A toner that remains on the surface of the electrophotographic photoreceptor by adhesion or fusion without being transferred onto a transfer member is spread on the surface of the electrophotographic photoreceptor in the form of a film while steps from charging to cleaning are repeated. This phenomenon corresponds to “adhesion wettability” in the wettability. Further, a phenomenon in which a paper powder, a rosin, talc or the like is adhered to the surface of the photographic photoreceptor and the contact area with the electrophotographic photoreceptor is then increased to provide strong wettability also corresponds to “adhesion wettability”.
FIG. 6 is a side view showing a state of adhesion wettability. In the adhesion wettability shown in FIG. 6, the relation between the wettability and the surface free energy (γ) is represented by Young's formula (1).γ1=γ2·cos θ+γ12  (1)wherein    γ1: surface free energy on a surface of product 1    γ2: surface free energy on a surface of product 2    γ12: interface free energy of products 1 and 2    θ: contact angle of product 2 to product 1
In formula (1), reduction in wettability of product 2 to product 1 which means that θ is increased for less wetting is attained by increasing the interface free energy Y12 related with a wetting work of the electrophotographic photoreceptor and the foreign matters and decreasing the surface free energies γ1 and γ2.
When adhesion of foreign matters, water vapor and the like to the surface of the electrophotographic photoreceptor is considered in formula (1), product 1 corresponds to the electrophotographic photoreceptor and product 2 to foreign matters respectively. Accordingly, when the electrophotographic photoreceptor is actually cleaned, the wettability on the right side of formula (1), namely, the adhered condition of the toner, paper powder and the like as foreign matters to the electrophotographic photoreceptor can be controlled by controlling the surface free energy γ1 of the electrophotographic photoreceptor.
In the prior technique that defines a surface condition of an electrophotographic photoreceptor, a contact angle with pure water is used (refer to, for example, Japanese Unexamined Patent Publication JP-A 60-22131 (1985)). However, in regard to wetting of a solid and a liquid, the contact angle θ can be measured as shown in FIG. 6, but in case of a solid and a solid such as an electrophotographic photoreceptor and a toner or a paper powder, the contact angle θ cannot be measured. Accordingly, the foregoing prior technique can be applied to wettability between a surface of an electrophotographic photoreceptor and pure water, but a relation between wettability and cleanability of a solid such as a toner constituting a developer or a paper powder cannot be explained satisfactorily.
With respect to an interface free energy between a solid and a solid which is deemed necessary for evaluation of a wettability between a solid and a solid, the Forkes 's theory stating a non-polar intermolecular force is considered to be further extended to a component formed by a polar or hydrogen-bonding intermolecular force (refer to Kitazaki T., Hata T., et al.; “Extension of Forkes's Formula and Evaluation of Surface Tension of Polymeric Solid”, Nippon Secchaku Kyokaishi, Nippon Secchaku Kyokai, 1972, vol. 8, No. 3, pp. 131–141). According to this extended Forkes's theory, the surface free energy of each product is found from 2 to 3 components. The surface free energy in the adhesion wettability corresponding to the adhesion of the toner or the paper powder to the surface of the electrophotographic photoreceptor can be found from 3 components.
The surface free energy between solid products is described below. In the extended Forkes's theory, an addition rule of the surface free energy represented by formula (2) is assumed to be established.γ=γd+γp+γh  (2)wherein    γd: dispersion component (non-polar wettability)    γp: dipolar component (polar wettability)    γh: hydrogen-bonding component (hydrogen-bonding wettability)
When the addition rule of formula (2) is applied to the Forkes 's theory, the interface free energy Y12 between product 1 and product 2 which are both solids is obtained as shown in formula (3).γ12=γ1+γ2−{2√(γ1d·γ2d)+2√(γ1p·γ2p)+2√(γ1h·γ2h)}  (3)wherein    γ1: surface free energy of product 1    γ2: surface free energy of product 2    γ1d, γ2d: dispersion components of product 1 and product 2    γ1p, γ2p: dipolar components of product 1 and product 2    γ1h, γ2h: hydrogen-bonding components of product 1 and product 2
The surface free energies (γd, γp, γh) of the components in the solid products to be measured as represented by formula (2) can be calculated by using known reagents and measuring adhesion with the reagents. Accordingly, with respect to product 1 and product 2 , it is possible that the surface free energies of the components are found and the interface free energy of product 1 and product 2 can be found from the surface free energies of the components using formula (3).
On the basis of the concept of the solid-solid interface free energy found in this manner, another prior technique controls wettability of a surface of an electrophotographic photoreceptor and a toner or the like using a surface free energy of the electrophotographic photoreceptor as an index (refer to Japanese Unexamined Patent Publication JP-A 11-311875 (1999). Still another prior technique discloses that a surface free energy is defined in the range of from 35 to 65 mN/m to improve cleanability of a surface of an electrophotographic photoreceptor and realize a long life thereof.
According to the present inventors' investigations, however, in the test of photography in which an image is actually formed on, for example, a recording paper using an electrophotographic photoreceptor having the surface free energy in the range disclosed in another prior technique, damage considered to occur by contact with foreign matters such as a paper powder and the like was confirmed on the surface of the electrophotographic photoreceptor. Further, it was also confirmed that owing to insufficient cleaning caused by this damage, black streaks occurred on images transferred on the recording paper. There was a tendency that the damage generated on the surface of the electrophotographic photoreceptor was increased with the increase in surface free energy.
In still another technique, an amount (Δγ) of change in surface free energy according to duration of an electrophotographic photoreceptor is defined. However, in consideration of the facts that the amount (Δγ) of change is not determined by defining initial characteristics, for example, the surface free energy, of the electrophotographic photoreceptor and the amount (Δγ) of change varies depending on conditions such as an environment in image formation and a material of a transfer member, the amount (Δγ) of change is problematic in that it might include an uncertain element and is therefore inappropriate as a designing standard in actual designing of an electrophotographic photoreceptor.